Sitak and Koolhof fought back from a break down in the second set before losing it in a tie breaker but came out strong in the deciding match tie break.
"Anything can happen in the super breaker but we were more aggressive and it felt like we wanted it more and in the end Wesley hit an unbelievable cross court winner to clinch the match." Sitak said.
Sitak and Koolhof will take on Poland's Marcin Matkowski and Sitak's former partner American Nicholas Monroe for a place in the semis tonight (9pm NZT).
Meanwhile New Zealand's top ranked singles player Rubin Statham's run has come to an end at the US $80,000 ATP Challenger tournament in Leon in Mexico.
Statham has lost 3-6,7-6 (5) 6-2 to Ecuador's Roberto Quiroz in the second round, his fifth match of the tournament after winning through three rounds of qualifying and his first round match.
Meanwhile Kiwi Erin Routliffe and her Chilean partner Alexa Guarachi Mathison have lost a marathon encounter in the ITF $80,000 tournament in Charlottesville, Virginia.
The second seeds have been beaten 6-2, 1-6 20-18 by American's Ingrid Neel, and Maria Sanchez.